Currently, in the development of high-frequency radio devices, the realization of low-loss connections for integration of various types of high-frequency circuits and the cost reduction and mass production of each element circuit are required. Therefore, the realization of the high frequency radio device within a small space while maintaining high-performance and high-functional characteristics is a key factor. In the high-frequency radio device, physical dimensions of a passive circuit such as a filter are substantially determined by a design frequency. Therefore, passive circuits such as filters have a low degree of freedom in flexible mounting components thereof.
A related band-pass filter is realized by sandwiching an E-plane parallel metal plate between rectangular waveguides obtained by dividing a rectangular waveguide in two at the middle of an H-plane to configure a single waveguide. In the case of the above-mentioned structure, the metal plate which is an element with high manufacturing accuracy and the rectangular waveguides which are subjected to cutting work are required. In terms of the connection to and the integration with planar circuits in a periphery, a mounting space is required.
Therefore, a technology of equivalently realizing a band-pass waveguide filter in a dielectric substrate has hitherto been proposed.
For example, in JP-A-11-284409 (Patent Document 1), there is disclosed a “waveguide band-pass filter” which has high productivity and can meet a requirement of size reduction. The waveguide band-pass filter disclosed in Patent Document 1 comprises a pair of main conductor layers sandwiching a dielectric substrate therebetween, and two rows of through conductor groups for sidewalls formed at intervals smaller than ½ of a signal wavelength in a signal transmission direction so as to electrically connect the main conductor layers. A plurality of through conductors, which electrically connect the main conductor layers to form inductive windows (inductive elements), are provided at intervals smaller than ½ of a wavelength in the waveguide in the signal transmission direction inside a dielectric waveguide line for transmitting a high-frequency signal through a region surrounded by the pair of main conductor layers and the two rows of through conductor groups for sidewalls.
In an example of one embodiment of Patent Document 1, as the plurality of through conductors, a maximum number (three in the embodiment) of through conductors are formed in an approximately middle portion of the dielectric waveguide line so as to be separated away from each other in a width direction. As the through conductors are separated away from the middle portion to both sides in the signal transmission direction, the number of the through conductors decreases.